Direction related interpolation is useful, for example. for generating additional lines for display in "line doubling" or "progressive scan" video displays. In progressive scan systems adjacent fields cannot be used to interpolate moving objects because motion blur will be introduced then. The most simple solution is to insert the average of the two adjacent lines for each missing line. This is known as vertical averaging and is also called "intra-field" interpolation because the average is based only on picture elements within a field.
A technique superior to vertical averaging represents what will hereinafter be called the "DIAG3" interpolation. An example of this form of interpolation is depicted in FIGS. 1a and 1b herein. Briefly stated, according to the found minimum gradient (absolute value is averaged over three pixels) the orientation of the interpolation filter is chosen. For simplicity of the hardware complexity only the two diagonal and the vertical direction are possible for an interpolation.
In more detail, in FIG. 1a the current pixel 103 is to be interpolated from pixels of the same field of an interlaced input signal, containing the upper adjacent line 101 and the lower adjacent line 102. The interpolation of the current pixel is related to the direction of a structure in the picture content.
As shown in FIG. 1b, three average values for the current pixel are calculated in vertical direction with a first averager 111, in diagonal ascending direction with a second averager 112 and in diagonal descending direction with a third averager 113. Three absolute difference values between adjacent pixel pairs of the two adjacent lines are calculated direction related in the respective branches with first absolute difference value means 121, 131 and 141, with second absolute difference value means 121, 132, and 142 and with third absolute difference value means 123, 133 and 143. The three absolute difference values of each branch are summed in a first adder 151, in a second adder 152 and in a third adder 153, respectively. The minimum of the three sums is detected in a minimum detector 16 and used in a selector 17 to select the average value of the respective branch for forming the current output pixel value 18.